loaded model classes have their connections wrapped in transactions.

See #17776

In Rails 4 config.eager_load was changed to false in the test environment. This
means that model classes that connect to alternate databases with
establish_connection are not loaded at start up. If use_transactional_fixtures
is enabled, transactions are wrapped around the connections that have been
established only at the start of the test suite. So model classes loaded later
don't have transactions causing data created in the alternate database not to
be removed.

This change resolves that by creating a new connection.active_record
notification that gets fired whenever a connection is established. I then added
a subscriber after we set up transactions in the test environment to listen for
additional connections and wrap those in transactions as well.